There is a long felt need for an economically viable process to form VAM directly from acetic acid. VAM is an important monomer in the production of polyvinyl acetate and polyvinyl alcohol products among other important uses. VAM is currently produced from two key raw materials, ethylene and acetic acid. Ethylene is predominantly produced from petroleum based raw materials although acetic acid can be produced to a lesser extent from petroleum based raw materials. Therefore, fluctuating natural gas and crude oil prices contribute to fluctuations in the cost of conventionally produced, petroleum or natural gas-sourced VAM, making the need for alternative sources of VAM all the greater when oil prices rise.
It has now been found that VAM can be produced essentially from a mixture of carbon monoxide and hydrogen (commonly known as synthesis gas or syn gas) involving a few industrially viable steps. For example, it is well known that synthesis gas can be reduced to methanol, which is in fact the industrially preferred way to manufacture methanol. Methanol thus formed can then be converted selectively to acetic acid under catalytic carbonylation conditions, which is again the industrially preferred process for the manufacture of acetic acid. The acetic acid thus formed then can be selectively converted to acetaldehyde under suitable catalytic conditions. The thus formed acetaldehyde along with acetic anhydride are reacted and converted to ethylidene diacetate which is subsequently thermally decomposed to VAM and acetic acid. Although there are no known preferred processes for such a conversion, the prior art does provide certain processes for such a conversion of acetic acid to acetaldehyde albeit at low conversions and yields thus making it industrially unsuitable.
For instance, the catalytic hydrogenation of aromatic carboxylic acids to produce aromatic aldehydes has been reported in the literature. For example, U.S. Pat. No. 4,613,700 to Maki et al. discloses that aromatic aldehydes can be formed from aromatic carboxylic acids using a catalyst comprising zirconium oxide containing as an essential component at least one element selected from the group consisting of chromium, manganese, iron, cobalt, zinc, bismuth, lead rhenium and the elements of Group III in periods 3 to 6 of the periodic table. However, no examples of catalytic hydrogenation of aliphatic carboxylic acids such as acetic acid are provided in this disclosure.
U.S. Pat. No. 5,306,845 to Yokohama et al. discloses a method of producing an aldehyde, which comprises hydrogenating a carboxylic acid or its alkyl ester with molecular hydrogen in the presence of a catalyst containing chromium oxide of high purity having a specific surface area of at least 10 m2/g and a total content of sodium, potassium, magnesium and calcium of not more than 0.4 weight percent. It is further reported therein that the hydrogenation reaction is conducted while maintaining the carboxylic acid or its alkyl ester at a concentration of not more than 10 volume percent. Additionally, the only example reported therein is hydrogenation of stearic acid to stearyl aldehyde. Most importantly, the selectivity to aldehyde drops significantly even if the total content of sodium, potassium, magnesium and calcium increases from about 0.3 weight percent to about 0.46 weight percent, thus rendering the process not suitable for a commercial operation.
U.S. Pat. No. 5,476,827 to Ferrero et al. describes a process for the preparation of aldehydes by catalytic hydrogenation of carboxylic acids, esters or anhydrides utilizing a bimetallic ruthenium/tin catalyst. The preferred carboxylic acids are the α-β-unsaturated carboxylic acids with an aromatic back bone or aromatic carboxylic acids. No examples of aliphatic carboxylic acids including acetic acid are provided.
U.S. Pat. No. 6,121,498 to Tustin et al. discloses a method for producing acetaldehyde from acetic acid. In this process, acetic acid is hydrogenated with hydrogen at an elevated temperature in the presence of an iron oxide catalyst containing between 2.5 and 90 weight percent palladium. However, the optimal condition reported therein is comprised of an iron oxide catalyst containing at least about 20 weight percent palladium, which affords about 80 percent selectivity to acetaldehyde with about 50 percent conversion of acetic acid. Additionally, significant amounts of by-products including methane, ethane, ethylene, ethanol and acetone are formed.
The acetaldehyde so formed may selectively be reacted with acetic anhydride to form ethylidene diacetate first and subsequent thermal decomposition of the ethylidene diacetate to VAM and acetic acid.
For example, U.S. Pat. No. 2,021,698 to Perkins teaches a process for making vinyl esters, such as vinyl acetate, by reacting acetic anhydride with paraldehyde and sulfuric acid. There is no teaching of making ethylidene diacetate first and then thermally decomposing said diacetate in order to obtain vinyl acetate and acetic acid. Perkins teaches away from the use of an intermediate product, such as ethylidene diacetate to arrive at vinyl acetate.
U.S. Pat. No. 4,843,170 to Isshiki et al. discloses a process for producing vinyl acetate from methanol wherein the methanol is converted to ethanol, methyl acetate, dimethylacetal, and acetaldehyde. The methyl acetate is further processed through carbonylation to form acetic anhydride which is mixed with the dimethylacetal and acetaldehyde from the methanol conversion step. The acetic anhydride, dimethylacetal and acetaldehyde are reacted to form ethylidene diacetate, which is thermally decomposed to form VAM and acetic acid. This multi-step process includes more stages than the process of the present invention in order to produce acetaldehyde.
U.S. Pat. No. 4,978,778 to Isshiki et al. also discloses a process for producing vinyl acetate wherein the acetic anhydride is reacted with hydrogen in the presence of a catalyst instead of acetaldehyde. As the described process converts vinyl acetate directly from acetic anhydride and hydrogen, there is no need to produce ethylidene diacetate in order to form VAM through thermal decomposition which is the basis for the present invention.
Alternatively, acetaldehyde may also be reacted with a ketene to form VAM. For instance, U.S. Pat. Nos. 5,719,315 and 5,531,456 both to Tustin et al. disclose processes for the preparation of vinyl acetate wherein acetaldehyde is mixed with a ketene and the mixture is subsequently contacted with a catalyst in a contact zone.
From the foregoing it is apparent that the existing processes do not have the requisite selectivity to form acetaldehyde directly from acetic acid and then react so formed acetaldehyde and acetic anhydride selectively to form ethylidene diacetate with further conversion through thermal decomposition to VAM and acetic acid in an integrated process thus making them industrially adoptable to produce VAM essentially from synthesis gas and/or synthesis gas based products.